This is a sample valid cache in 2.0 format.
This was produced before the 3.0 format change by the following test:

TEST_F(DiskCacheBackendTest, CreateCorrect) {
  const char* kKey = "https://example.org/data";
  cache_path_ = base::FilePath("/tmp/cache-correct-2.0");
  CleanupCacheDir();
  CHECK(base::CreateDirectory(cache_path_));

  SetMaxSize(16*1024);
  InitCache();

  disk_cache::Entry* entry = nullptr;
  ASSERT_THAT(CreateEntry(kKey, &entry), IsOk());

  const int kBufSize = 1234;
  auto buffer = base::MakeRefCounted<net::IOBufferWithSize>(kBufSize);
  CacheTestFillBuffer(buffer->data(), kBufSize, /*no_nulls=*/false);

  EXPECT_EQ(kBufSize, WriteData(entry, /*index=*/1, /*offset=*/0, buffer.get(),
                                /*len=*/kBufSize, /*truncate=*/false));
  entry->Close();
}

With the following patch applied:
--- a/net/disk_cache/blockfile/block_files.cc
+++ b/net/disk_cache/blockfile/block_files.cc
@@ -487,7 +487,7 @@ bool BlockFiles::GrowBlockFile(MappedFile* file, BlockFileHeader* header) {

   ScopedFlush flush(file);
   DCHECK(!header->empty[3]);
-  int new_size = header->max_entries + 1024;
+  int new_size = header->max_entries + 64;
   if (new_size > kMaxBlocks)
     new_size = kMaxBlocks;

To keep the data_ file size down somewhat.